1. Field of the Invention
This invention provides an apparatus for drying a travelling web which may, for example, be a web of paper or paperboard.
2. Description of the Prior Art
The introduction of various new methods of forming paper and board in the last ten years has led to significant increases in paper machine speed and it is now possible to manufacture paper at more than twice the speeds achieved previously using traditional forming methods. Significant improvements in press design have been made over the corresponding period as the machine speed increased but the moisture content of the fibre web entering the dryers has not changed appreciably. Greatly increased demands are therefore made on the dryer section, especially in the manufacture of paperboards, and the cost of the increased number of steam heated cylinders together with the extended building and ancilliaries has become a major capital item. Further, in many older machines, the full benefit of rebuilding the pressing and forming section cannot be realized due to drying limitations.
Much effort has been expended in attempts to develop drying techniques to supplement or replace the steam heated cylinders. The list would include sectional and pocket ventilation, high velocity air impingement hoods, and plenums acting against the unsupported sheet, radiant and high frequency systems, vacuum and through drying techniques, fluidized bed and low melting point alloy systems.
These various techniques are well documented and it is fair to say that none are as satisfactory, overall, as the steam heated cylinder when properly ventilated. Condensate removal systems for cylinders have kept pace with machine speed increases and the economics of simultaneous power and steam generation is such that cylinder drying is not equalled by any alternative system except perhaps by the Papridrier, which is not yet proved for heavier paperboard manufacture. The various alternatives have only found limited application, to supplement output where costing is on a marginal basis, to eliminate moisture streaks, dry paper coatings and exploit any anomalies in fuel costs. For lightweight papers the air impingement hood is often employed in conjunction with a Yankee cylinder.
The prospects for advance in drying technology seem to be best where the process is based on the present conventional steam cylinder system.
Many aspects of the drying mechanism on the cylinder are still debated. However, it is indisputable that less than half the board surface is in contact with the cylinders at any time. It is equally certain that air impingement onto the remaining available surface will significantly increase the rates of evaporation. The extent depends on a number of factors, in particular the thickness of the board. In practice, however, air impingement is more difficult and expensive to arrange on multi cylinder dryers than say on a Yankee tissue machine where only one large cylinder is usually employed. Firstly, a whole series of hoods are required which are costly and occupy much space and the broke arising from breaks in the sheet accumulating in each hood can have a disastrous effect on machine efficiency. Attempts to use impingement dryers on individual steam cylinders of paperboard machines have been largely abandoned for the above reasons.
In principle, the air impingement technique remains attractive since evaporation rates of 2 to 8 times the rate for cylinders alone can be achieved. The present invention provides an apparatus which enables air impingement to be achieved without the mechanical and other limitations of fixed air impingement hoods.